With the evolution of technology, electronic devices having different kinds of functions are developed. Those electronic devices may be configured to have their own power supply standards. A power bank is a portable device designed for charging an electronic device. However, when all electronic devices have their own power banks, it may cause inconvenience for their users. To solve this problem, a conventional power bank capable of outputting a plurality of predetermined output powers is developed for the electronic devices. These predetermined output powers, however, are not adjustable. If an electronic device with a required power not covered by the predetermined output powers is plugged into the conventional power bank, the electronic device would not be powered up by the output power, and the output power may even destroy the electronic device.